In-situ Acoustic Velocity Measurement on Mantle Phase Transitions

地幔中的相变所造成的压力和温度的变化，随着深度的增加，被认为是负责在地震研究中观察到的速度不连续性。在这个项目中，在多砧装置中使用集成的超声干涉测量和同步辐射X射线辐射技术，研究了在与地球地幔相关的P-T条件下经历相变的候选地幔相的弹性。这些实验允许同时测量晶格参数（因此特定的体积和密度）和P和S波速度跨越地幔相的相变高达26 GPa和1800 K。要调查的相变包括橄榄石wadsleyite，wadsleyite ringwoodite，和ringwoodite钙钛矿加镁橄榄石的解离，被认为是主要负责在410-，520-，和660公里深度的地震不连续性。具有重要意义的是，通过直接测量的速度与热解组合物的多相聚集体，强大的速度对比跨越这些相变可以在物理和化学条件下，这些不连续性在地幔中。可以消除来自于计算假设混合骨料的物理性质的复杂性的不确定性，例如使用各个相的状态方程、铁分配变化及其对弹性和相平衡的影响的外推。因此，目前的结果与地震数据的比较将放置一个严格的约束候选地幔成分负责观察到的幅度和横向变化在这些不连续性。

Phase transitions in the mantle resulting from pressure and temperature changes with increasing depth are believed to be responsible for the velocity discontinuities observed in seismic studies. In this project, elasticity of candidate mantle phases undergoing phase transitions at P-T conditions relevant to the Earth's mantle are studied using an integrated ultrasonic interferometry and synchrotron X-radiation technique in the multi-anvil apparatus. These experiments allow for simultaneous measurements of lattice parameter (hence specific volume and density) and P and S wave velocities across the phase transitions of mantle phases up to 26 GPa and 1800K. The phase transitions to be investigated include olivine to wadsleyite, wadsleyite to ringwoodite, and the dissociation of ringwoodite perovskite plus magnesiowustite which are believed to be predominantly responsible for the seismic discontinuities at 410-, 520-, and 660-km depths. Of great significance is that, by directly measuring the velocities of multi-phase aggregates with pyrolitic compositions, robust velocity contrasts across these phase transitions can be derived at the physical and chemical conditions relevant to these discontinuities in the mantle. The uncertainties originating from the complexities in the calculation of physical properties for a hypothetically mixed aggregate, such as extrapolations using equation of state for individual phases, iron partitioning change and their effect on elasticity and phase equilibrium, can be eliminated. As a result, the comparison of the current results with seismic data will place a tight constraint on the candidate mantle compositions responsible for the observed magnitude and lateral variations at these discontinuities.